206 



MECHANICAL SYSTEM 



attempts to draw the fingers upwards along the stem, the friction 

 experienced is often sufficient to arrest the movement altogether. 

 The explanation is very simple> consisting in the fact that the barbs of 

 the climbing-hairs all point downwards. The climbing-hairs of Phaseo- 

 lus vulgaris and P. multifiorus are exceptions to the rule, since they 

 are not arranged in any definite manner. They do indeed often point 

 downwards (i.e. towards the base of the stem), but just as often have 

 their tips directed upwards or even sideways. This aberrant behaviour is 

 probably connected with the fact that the hairs are somewhat flexible, 



instead of being rigidly attached to the stem. The 

 cell which forms the body of the hair is thin-walled 

 towards the base, where it is inserted upon a stiff 

 thick-walled stalk-cell ; the hooked body-cell can 

 therefore twist and turn in various directions around 

 the rigid stalk-cell (Fig. 76 d). 



The simplest type of climbing-hair consists of a 

 single cell, which ends in a strongly recurved sharply 

 pointed tip : the thickness of the cell-wall increases 

 towards the point (Galium aparine, Fig. 76 b). In 

 Humulus Lupulus the two-armed grappling-hair is 

 oblicpiely inserted upon a multicellular pedestal or 

 emergence, in such a way that its distance from 

 the surface is greatest on the basiscopic side (Fig. 

 76 a). A very effective arrangement is found in 

 the Loasaceae, where the unicellular climbing-hairs 

 bear several whorls of minute but very sharp 

 barbs : these originate as tiny protuberances of the 

 cell-wall, but when fully developed take the form of massive calcified 

 projections (Fig. 76 e). 



Closely related to these trichomes of climbing plants are the 

 remarkable anchoring-hairs of Avicennia officinalis, first discovered by 

 Karsten. 111 These structures, which serve to fix the seedlings in the 

 mud when they first fall from the tree, occur in great numbers 

 on the hypocotyl. Each consists of a row of elongated cells, the 

 side walls of which become constantly thicker towards the apex 

 of the hair. The terminal cell is strongly recurved, and ends in 

 a sharp point (Fig. 77). The hairs are freely movable in all 

 directions, the thin-walled basal cells acting like the cable of a ship's 

 anchor; in the long run, this arrangement greatly increases the 

 chances of fixation. 



A very similar anchoring device has been observed by Fritz 

 Miiller 11 - in the seeds of a Bromeliaceous epiphyte, Catopsis nutans, 

 which almost always attaches itself to the very outermost and thinnest 



Fig. 77. 



Anchoring-hairs from 

 the hypocotyl of Avicen- 

 nia officinalis. 



